1. Field of the Invention
The present invention relates to a diesel engine, particularly to a control of fuel injection timing and EGR recirculation quantity.
2. Description of the Related Art
In a diesel engine that performs a lean operation mainly caused by diffusion combustion, NOx is likely to occur by a high excess air ratio. As its countermeasure, an EGR control is used in which a part of exhaust gas is recirculationed into an intake air system and a combustion temperature is decreased to suppress the generation of NOx. Though the NOx suppression effect is enhanced along with an increased EGR ratio, in the diffusion combustion in which combustion is caused on a combustible gas mixture layer at the boundary between injection fuel and compressed air, a cinder of fuel occurs if the EGR ratio is excessively increased, resulting in a problem that the smoke emission amount is rapidly increased. Therefore, to avoid this problem, the EGR ratio had an upper limit value, so that the NOx suppression could not be fully realized.
In JP-A-8-218920 (U.S. Pat. No. 5,732,554), a technique for discharging NOx from the NOx occluded substance provided in an exhaust system of the diesel engine was disclosed. Fuel is injected in a suction stroke and a large quantity of EGR gas is introduced to lower the excess air ratio at or below 1.0 thereby exhausting the NOx. The combustion state is changed to a premix combustion in which an injected fuel is diffused and vaporized before the top dead center of compression, premixed with the air and combusted, whereby the cinder of fuel is less likely to arise, causing the upper limit value of the EGR ratio to be raised. Thus, it is presumed that the smoke and NOx can be both reduced.
On the other hand, JP-B-3116876 (U.S. Pat. No. 5,937,639) discloses a technique for controlling the EGR ratio when the smoke emission amount tends to decrease in a predetermined area such as a low load condition in order to reduce both of the smoke and NOx. It is noted that if the EGR ratio is increased above the upper limit value, the smoke emission amount tends to decrease after rapidly increased to the peak.
However, with the first technique, since there is a long interval from fuel injection in a suction stroke to ignition in a compression stroke, there is a problem that the ignition timing is varied, likely causing an ignition failure such as preignition or ignition delay and a degradation of stability. Also, a part of the fuel injected in the suction process is diffused within a cylinder and deposited on a cylinder wall to cause an oil dilution, and is not captured within a cavity of a piston like the fuel injected at a timing of the top dead center of compression, resulting in a problem that hydrocarbon (HC) or carbon monoxide (CO) is rapidly increased.
Also, with the second technique, when the control mode is switched between the predetermined area and the other areas, the peak of the smoke emission amount appears. Thus, there is a problem that the smoke emission amount is necessarily rapidly increased transiently at every time of mode switching.
It is an object of the present invention to provide a diesel engine that can reduce both of smoke and NOx by increasing an upper limit value of EGR ratio for suppressing emission of smoke while preventing oil dilution or increase in HC or CO from occurring.
In order to achieve the above object, the present invention provides a control device for a diesel engine wherein a fuel injection timing by a fuel injection part for injecting a fuel into a combustion chamber of the engine is set at an advance side from an injection timing when the smoke emission characteristic indicates a decreasing tendency after indicating an increasing tendency in accordance with an increase in an EGR ratio, and a lag side from an injection timing when the injected fuel gets out of a cavity formed in a piston and gets to a cylinder wall face, and an EGR ratio adjusting part for adjusting a recirculation amount of exhaust gas exhausted from the engine into an intake air system is operated to reduce the smoke emission amount and the NOx emission amount from the engine at the same time.
The smoke emission characteristic for the EGR ratio is changed depending on the injection timing. FIG. 5 shows an example of the results of a test in which the fuel injection timing is changed in a certain operating range. If the injection timing is advanced to 20xc2x0 BTDC as compared with 10xc2x0 BTDC that is applicable to the typical diesel engine, the smoke emission amount for an increase in the EGR ratio indicates an increasing tendency up to a peak, and then a decreasing tendency. In this case, if the excess air ratio is changed with the EGR ratio, the smoke emission amount is over the peak, whereby it is impossible to avoid a rapid transient increase of the smoke emission amount. On the contrary, at 36xc2x0 BTDC to which the injection timing is further advanced, the smoke emission amount is suppressed to the low value, even if the EGR ratio is increased, and the NOx emission amount is suppressed by the high EGR ratio, whereby the smoke and NOx can be reduced at the same time, and the transient increase of smoke is suppressed because no peak of the smoke emission amount is formed.
In this manner, the smoke emission amount is suppressed by the advanced injection timing, because the period from fuel injection to ignition is extended to promote the premix between injected fuel and intake air. In a suction stroke injection, for example, the injection timing is not extremely advanced, so that the injected fuel is surely ignited at a predetermined timing neat the top dead center of compression, and the stable operation is enabled without ignition failure.
On the other hand, FIG. 6 shows the test results of measuring the THC emission amount and the dilution fuel amount by changing the injection timing in the same operation range as in FIG. 5. If the injection timing is advanced, THC is increased from around 40xc2x0 BTDC, and the amount of fuel mixed into the engine oil is increased, bringing about a danger of oil dilution.
In view of the above main causes, if the fuel injection timing is set at an advance side from the injection timing when the smoke emission characteristic indicates an increasing tendency and then a decreasing tendency for an increase in the EGR ratio, and a lag side from the injection timing when the injected fuel gets out of a cavity formed in a piston and gets to a cylinder wall face, the problem with the increased THC or oil dilution due to excessive advance is prevented, and the upper limit value of the EGR ratio to suppress the exhaust of smoke can be increased. And since the recirculation amount of exhaust gas is controlled on the basis of this EGR ratio by the EGR adjusting means, the smoke emission amount and the NOx emission amount can be reduced at the same time.
In a preferred form, the injection timing is advanced along with the increase in the engine rotational speed.
In this case, if the piston speed is increased with the engine rotational speed, the timing when the injected fuel get into the cavity occurs relatively earlier. Therefore, to bring the injected fuel into the cavity at an appropriate timing, it is required to expedite the fuel injection up to the time when the piston position is still low. Hence, if the injection timing is advanced with the increase of the engine rotational speed, the injected fuel is always brought into the cavity of the piston at appropriate timing, irrespective of the engine rotational speed, whereby a combustible state consistent with the smoke and NOx is stably realized.
Also, the injection timing is desirably kept almost constant for a change in the load.
In this case, since the dependency of the injection timing on the engine load is lower than on the engine rotational speed, if the injection timing is kept almost constant for a change in the load, the combustible state consistent with the smoke and NOx can be stably realized without being affected by the variation in the engine load.
Preferably, the operation of the EGR ratio adjusting means is controlled so that the EGR ratio may be 50% or greater and the excess air ratio may be 1.0 or more.
In this case, if the EGR ratio is 50% or greater, NOx can be reduced efficiently owing to a large amount of EGR, and if the excess air ratio is 1.0 or more, the exhaust of HC or CO can be effectively suppressed. Consequently, the comprehensive emission exhaust characteristic can be improved.
When a catalyst having an oxidation function is provided in the exhaust system, it is preferable that execution of the invention or the first control mode is inhibited when the catalyst is inactive.
Accordingly, a large amount of EGR in the first control mode leads to decreasing excess air, so that the emission amount of HC or CO tends to increase, but the exhausted HC or CO is surely purged due to the oxidation function of catalyst. On the other hand, when the catalyst is inactive, execution of the first control mode is inhibited, so that HC or CO exhausted from the engine is decreased with the smaller EGR amount, thereby preventing the exhaust of HC or CO.
Moreover, when a second control mode is provided in which the injection timing is set near the top dead center of compression to lag behind the first control mode, and the EGR ratio is lower than in the first control mode, the injection timing is momentarily switched in the middle during an EGR ratio change period where the EGR ratio is gradually changed, when switching between the first control mode and the second control mode depending on the operating conditions of the engine.
In the first control mode, though the problem with the increasing HC or CO and the oil dilution as previously described is prevented, the upper limit value of the EGR ratio to suppress the exhaust of smoke is increased, so that the smoke emission amount and the NOx emission amount can be reduced at the same time.
Also, in the second control mode, in contrast to the first control mode, because the injection timing is lagged and the EGR ratio is smaller, the mode switching means switches the control mode into the second control mode to cope with an output requirement in an operating condition with high output requirement from the driver.
On the other hand, when the control mode is switched by the mode switching means, the EGR ratio is gradually changed due to recirculation of the exhaust gas, but if the injection timing is gradually changed in accordance with the change characteristic, the smoke may be temporarily increased in a state where the injection timing and the EGR ratio are in the range for rapidly increasing the smoke.
For example, FIG. 4 shows the test results in which the injection timing is further subdivided in the same operating range as in FIG. 5. In FIG. 4, the characteristic is indicated with a sign xe2x97xaf for IT=8xc2x0 BTDC. In the same manner, the characteristic is indicated with sign ◯ for 10xc2x0 BTDC, sign ▴ for 12xc2x0 BTDC, sign xcex94 for 14xc2x0 BTDC, sign x for 16xc2x0 BTDC, sign ★ for 20xc2x0 BTDC, sign ♦ for 24xc2x0 BTDC, sign ⋄ for 28xc2x0 BTDC, sign ▪ for 32xc2x0 BTDC, and sign xe2x96xa1 for 36xc2x0 BTDC.
In FIG. 4, the second control mode is indicated with a large sign xe2x97xaf, in which the injection timing is set at 8xc2x0 BTDC, the EGR ratio Regr is set at 45%, and the excess air ratio xcex is controlled to be around 1.8. On the contrary, the first control mode is indicated with a large sign xe2x96xa1, in which the injection timing is set at 36xc2x0 BTDC on the far advance side, and the EGR ratio Regr is set at 56%, so that the excess air ratio xcex is controlled to be 1.0 or on the slightly lean side.
When the injection timing is set somewhere from 14 to 32xc2x0 BTDC in a transient range of the excess air ratio xcex from 1.0 to 1.8, there occurs a phenomenon that the noise is increased mainly on the lean side, or the smoke is increased mainly on the lean side. Hence, the noise and the smoke are inconsistent. Accordingly, when the control mode is switched between the first control mode and the second control mode, if the injection timing is gradually changed in accordance with the broken line of FIG. 4, the range of 14 to 32xc2x0 BTDC is necessarily passed, whereby it is not possible to avoid the region where the noise or the smoke is rapidly increased.
On the contrary, when the injection timing is set at 8 or 10xc2x0 BTDC, the noise and the smoke can be reduced in the region on the lean side from the excess air ratio xcex of about 1.3. On the other hand, when the injection timing is set at 36xc2x0 BTDC, the noise and the smoke can be reduced in the region on the rich side from the excess air ratio xcex of about 1.3. Hence, in this example, the injection timing may be changed momentarily at the excess air ratio xcex=1.3, as indicated by the solid line in FIG. 4.
In the above example, since the injection timing is momentarily switched in the middle during an EGR ratio change period where the EGR ratio is gradually changed, the control mode is switched to pass over the region where the smoke is rapidly increased. Consequently, it is possible to avoid beforehand a temporary increase of the smoke during the EGR ratio change period.
The first control mode is preferably executed when the operating range of the engine is in a low load region, whereby the control can be performed without trouble in the low load region where the required fuel amount is smaller even if the amount of air is decreased due to a large amount of EGR.
In another preferred form, when the engine has an NOx catalyst for occluding NOx in the exhaust system, it is preferable that the operation of the fuel injection means and the EGR ratio adjusting means is controlled so that the fuel injection timing may be almost equal to that of the first control mode, and the excess air ratio may be less than 1.0 at a time of purging NOx from the NOx catalyst.
In this case, since the fuel injection timing is set up at an advance side from the injection timing when the smoke emission characteristic indicates an increasing tendency and then a decreasing tendency for an increase in the EGR ratio, the range of the EGR ratio to maintain the smoke emission amount at a low level can be extended. Moreover, since the operation of the EGR ratio adjusting means is controlled so that the excess air ratio maybe 1.0 or less, the EGR ratio can be increased without worrying about the smoke emission amount to discharge and reduce NOx from the NOx occluded catalyst efficiently.